When the operational disturbance remains, the pressure has to be reduced by removing combustion gases from the combustor and compressed combustion air from a pressure vessel surrounding the combustor.
Pressure reduction may, for example, be effected by simultaneous reduction of the pressure in the combustor and blow-off of combustion air from the pressure vessel to the atmosphere, such as described in U.S. Pat. No. 4,744,212 (Anderson et al) or in some other way.
When reducing the pressure in this way, the temperature of the bed material is insignificantly reduced and must be additionally cooled down to a temperature which eliminates the risk of sintering together of bed particles, or to room temperature. At temperatures above about 600.degree. C. and at the presence of oxygen, the fuel present in the bed involves a risk of the formation of carbon oxide for explosion.